This invention pertains broadly to the field of antennas. More particularly, the invention pertains to directional antennas.
For some applications it is desirable to transmit a signal in all directions equally, but for certain applications it is desirable to direct a signal in a specific direction and avoid radiation in other directions. This is particularly true in radio applications and even more so in military radio applications. All antennas are directional to a degree, that is, antennas will radiate more strongly in certain directions than others. Directional antennas, however, are designed to enhance this feature.
Some directional antennas depend upon radiation from two or more antenna elements added vectorially. If the waves radiated from the elements add in a particular direction, the signal will be strong in this direction, while if they tend to cancel or subtract in a certain direction, a signal will be nonexistent or weaker in that direction.
It is well known that the combination of a loop antenna and a monopole antenna can give a single or mono-directional antenna pattern. This is a common configuration used in radio direction finding. The loop antenna itself is a simple form of a directional antenna. FIG. 1A contains the pattern of a magnetic dipole (loop) antenna. This figure eight radiation pattern is constructed by drawing the magnitude of the power radiated by the antenna in all directions around a circle so that the distance from the center represents the magnitude of the signal in each direction. The directional pattern of the loop antenna may be altered by adding the radiation from a separate monopole (vertical) antenna. The radiation pattern of this monopole antenna is omnidirectional as shown in FIG. 1B.
FIG. 2 contains several combinations of the monopole and loop antenna elements. For all cases shown, the elements are assumed to be in-phase and all that is varied is the relative radiation magnitude of the antenna elements. The case in which the maximum value of the magnetic dipole (loop) antenna is equal to that of the monopole (vertical) antenna will give a perfect cardioid, as shown in FIG. 2A. This pattern is only achieved exactly at one elevation angle, namely the angle at which the radiation from the monopole is equal to the radiation from the magnetic dipole. The radiation is greatly reduced at other elevations in the direction of the cardioid null.
In the prior art there exists a radio direction finder described in U.S. Pat. No. 1,924,408 issued to August Leib. Leib shows a mast mounted shielded loop antenna with the mast and shield forming a monopole antenna. Because Leib's antenna is designed solely for receiving, the critical issues of feeding and isolating the two antenna elements are not addressed. Leib does not discuss feeding his loop antenna in a balanced manner, nor does he address the matter of isolation required for operating a dual-feed antenna at transmitting voltage levels.
An antenna for direction finding having mast isolation is described in U.S. Pat. No. 3,882,506 issued to Kenzo Mori et al. This antenna includes a monopole antenna that extends through and above two crossed-loop antennas. Transformers are used for both the loop and monopole feeds. In this invention, the shields of the loop antennas are not part of the monopole antenna system. Further, Mori was not concerned with high voltages as his invention is strictly for receiving direction finding. This is apparent as his vertical antenna must pass through a high impedance portion of his loop antennas, specifically the gap at the top of the loop antennas. This antenna, if used for transmitting, will lead to very high voltages at this point with the possibility of voltage flashover from one antenna to the other.
In yet a third scheme, Alan Carr has described a three-element antenna formed of orthogonal loops mounted on a monopole in U.S. Pat. No. 4,433,336. Carr's invention includes a base-insulated monopole antenna mast upon which crossed loops are mounted. The loops of Carr's invention are not shielded. In Carr's invention, the feed line for his loop antennas must pass across the feed point for his monopole antenna. This monopole feed point can be at very high voltage if the monopole is operated well below self-resonance. Isolation therefore must be provided to keep the loops from effectively shorting out the monopole antenna feed. Carr's invention uses a double ferrite choke on the outside of a coaxial cable to provide such isolation, however such chokes are known to be poor isolators especially in frequency ranges of interest to the United States Navy (VLF-HF or approx. 3 kHz to 30 MHz). In addition, Carr's invention uses a small loop inside a larger loop as a feed. This type of feed has the advantage of providing an impedance transformation, however, it is not a true balun as the shield of the coax feeding the loop of the antenna is directly connected to the monopole antenna structure. This will cause current to flow on the outside of the loop feed coax which will affect both impedance and radiation patterns of the antenna in ways difficult to predict.